<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 200px;
            height: 200px;
            background-color: red;
        }
        /*div.a.b{*/
            /*background-color: gray;*/
        /*}*/
        div.a{
            background-color: rgba(255,255,0,0.5);
        }
        /*#aa{*/
            /*background-color: lightpink;*/
        /*}*/
    </style>
</head>
<body>
    <div class="a b" id="aa">aaa</div>
    <div>bbb</div>
    <div>ccc</div>
    <div>ddd</div>
</body>
</html>